1. Field of the Invention
The present invention relates to radiation curable compositions suitable for UV LED curing, comprising at least one specific carbazole based initiator and an amine co-initiator.
2. Description of the Related Art
Photoinitiators are frequently used in polymerizable compositions, such as UV-curable inks, to initiate the polymerization of monomers when exposed to UV radiation. Bathochromic photoinitiators, absorbing in the region between 365 nm and 395 nm, are required to make full use of the recent development of UV-LEDs with increasing power. Thioxanthones and acyl phosphine oxides are photoinitiators absorbing in this spectral region.
Thioxanthones are prone to yellowing upon exposure, thereby forming degradation products with a limited stability. As a result, the original yellowing shifts upon storage. Especially in imaging, e.g. inkjet printing, this unstable yellowing behaviour makes control of the image tone in the final image difficult. On top of that, certain applications, predominantly packaging applications, prefer thioxanthone free radiation curable compositions.
Acyl phosphine oxides, on the other hand, result in medium volatile aldehyde type of degradation products, resulting in a background smell of the printed image, which is unacceptable in packaging applications.
Therefore, there is an increasing demand for the development of new photoinitiators, absorbing in the region between 365 nm and 395 nm, having a stable yellowing behaviour without generating medium volatile degradation products. Recent evolutions in bathochromic photoinitiators are based on carbazole derivatives.
Much effort in carbazole based initiators has been directed towards the development N-acyl oxime derivatives of bis ketocarbazoles as photoinitiators for black resists as recently reviewed by Dietliker et al. (Progress in Organic Coatings 58, 146-157 (2007) and disclosed in WO 2008/138733 (CIBA), WO 2008/138732 (CIBA), WO 2008/138724 (CIBA), WO 2007/071497 (CIBA), WO 2007/062963 (CIBA), WO 2006/018405 (CIBA), WO 2004/050653 (CIBA), WO 02/100903 (CIBA), WO 2008/075564 (MITSUBISHI CHEMICAL) and WO 2006/059458 (ASAHI DENKA).
Carbazole based Norrish type I initiators have been disclosed in JP 2007-254701 (TOYO INK), US 2003199601 (SAMSUNG ELECTRONICS), JP 63-168403 (FUJI PHOTO FILM) and EP 284561 A (CIBA)).
Bis-keto-carbazoles have been disclosed in photochemical applications as sensitizers for acyl oxime and oxime based photoinitiators in negative resist application (JP 2007-219362 (TOYO INK)) and radiation curable applications JP 2007-112930 (TOYO INK) and JP 2005-187678 (TOYO INK)). They have further been disclosed as sensitizing agents for cationic radiation curable formulations in US 2005113483 (KONICA MINOLTA), JP 2005-343847 (TOYO INK), JP 2000-239648 (JSR) and Yamamura et al., Journal of Photopolymer Science and Technology, 13(1), 117-118 (2000)) and JP 2001-109142 (JSR)).
JP 2005-113043 (KONICA) discloses a cationically UV curable inkjet ink composition containing an aliphatic tertiary amine and N-ethyl-3,6-bis(benzoyl)-carbazole photoinitiator. JP 2005-113043 (KONICA) is silent on free radical UV curable inkjet ink compositions.
JP 2006-162784 (TOPPAN PRINTING) discloses a photosensitive colored composition including a photopolymerization initiator, a photopolymerizable monomer, a resin, a pigment and a solvent, wherein a oxime ester compound is used as the photopolymerization initiator. Also EP 1395615 (CIBA) AND EP 1567518 (CIBA) disclose UV curable compositions including oxime ester compounds as the photopolymerization initiator.
The carbazole initiators, disclosed in the prior art often require multistep synthesis and are often still to hypsochromic to be cured by LED curing. Therefore, there is still a need for easy accessible photoinitiators exhibiting high curing speed upon LED exposure.